The present invention relates generally to footwear midsoles, and more particularly to shock absorbing midsoles, including heel cups and shanks, for athletic footwear.
Athletic footwear generally comprises a sole and an upper. The sole may be a single piece, but more commonly comprises multiple layers, namely, an outsole, insole and midsole there between. The outsole provides a tough, wear resistant layer and suitable tread for providing traction against the ground or floor. The insole is a thin, soft layer, and typically provides for comfort. The midsole is the primary structural layer in the sole, and provides for shock absorption.
Persons skilled in the art of athletic footwear design have endeavored to improve midsole shock absorption, especially shock caused by heel strike, while maintaining the structural integrity of the shoe and providing an adequate level of cushioning for comfort. Various solutions have been proposed and used. For example, U.S. Pat. No. 4,614,046 assigned to Puma Sportschufabriken Rudolf Dassler KG and U.S. Pat. No. 4,364,188 assigned to Wolverine World Wide, Inc. disclose running shoes having midsoles comprises of elastomeric foam. The midsoles have several zones or areas where elastomeric foam of different stiffness is used to balance the need for shock absorption and stability. Different types of elastomeric foams are disclosed, including ethylene-vinyl acetate (xe2x80x9cEVAxe2x80x9d), polyethylene, and polyurethane foams.
In an effort to improve on shock absorption, midsoles incorporating air bags have been developed. For example, U.S. Pat. No. 4,871,304 assigned to Nike, Inc., discloses a combination of a gas filled bladder within elastomeric foam materials. The combination of air bags with foam was intended to provide a mechanism for adjusting the impact response characteristics of the sole to desired requirements, such as the requirements for a particular sport. In a somewhat similar vein, U.S. Pat. No. 4,535,553, assigned to Nike, Inc. discloses an athletic shoe midsole that features a pattern of discrete, spaced, plastic shock absorbing projections in combination with elastomeric foam. Finally, U.S. Pat. No. 5,343,639 to Nike, Inc. discloses an athletic shoe midsole having plural columns disposed between upper and lower plates in the heel region of the shoe. The columns are elastomeric foam tubes, and include gas bladders disposed in hollow regions within the columns.
A drawback of most prior art midsoles is that they return a substantial portion of the energy of foot strike, which can be detrimental to the athlete. Specifically, most midsoles comprised of elastomeric foam and/or air bags, act as springsxe2x80x94storing the energy from foot strike while under compression, which may be returned immediately. This action may be referred to as energy return or rebound. There are those skilled in the art that hold the view that energy return is desirable. However, it has been found that rebound can produce undesirable shock to the athlete, and can in some cases lead to injury.
Another design objective of athletic footwear is stability. It is common among runners, particularly in amateur athletics, to land on their heels. More specifically, the foot strike occurs on the lateral side of the heel. As the athlete""s body moves forward, weight is transferred progressively forward and towards the medial side of the foot. The foot leaves the ground (xe2x80x9ctoe offxe2x80x9d) with the runner""s weight on the medial side of the foot. This rolling motion from lateral-heel to medial-toe is known a xe2x80x9cpronation.xe2x80x9d It is common in some athlete""s to detrimentally over pronate. To counteract over pronation, it is know to provide stabilizers in athletic shoe midsoles. For example, U.S. Pat. No. 4,614,046 to Puma Sportschufabriken Rudolf Dassler KG and U.S. Pat. No. 4,364,188 to Wolverine World Wide, Inc. disclose midsole designs where harder, less compressible materials are inserted or otherwise disposed on the medial sides of the heel midsole to control over pronation.
Structural integrity, torsional stability and ach support are yet other design considerations in footwear, including athletic footwear. U.S. Pat. No. 6,061,929 to Deckers Outdoor Corporation discloses a midsole with an integrally molded shank for providing torsional rigidity and arch support. It is also known to be beneficial to provide a heel cup, to surround and protect the athlete""s heel.
Although the foregoing efforts have met with varying degrees of success, there remains an unresolved need for a midsole for athletic footwear with improved shock absorption, stability and structural integrity.
It is an object of the invention to provide footwear midsole that has improved shock-absorbing performance in the heel region of the footwear. More specifically, it is an object to provide superior shock absorption, while minimizing undesirable rebound.
It is a second object of the invention to provide footwear having improved stability, especially torsional rigidity and over pronation control.
It is a third object of the invention to provide an innovative and attractive athletic shoe.
The foregoing objects are met by the midsole of the invention. The midsole comprises a heel cup having a semi-rigid upper plate, a lower plate composed of a material that is softer than the upper plate, and a plurality of telescopic shock absorbers between the upper and lower plates.
The midsole preferably comprises four spaced apart shock absorbers in the heel portion of the midsole. The telescopic shock absorbers have one portion that collapses into a second portion when the shock absorber is loaded or compressed. The shock absorbers are constructed to absorb significant compressive loads from heel strike during athletic use. On release of the load, however, the shock absorbers of the invention return only a controlled portion of the compressive load originally applied. More specifically, the shock absorbers have the capacity to absorb a predetermined maximum load, which is based on a variety of factors including the particular sport and the anticipated weight of the athlete. On release of the compressive load, a predetermined portion of the original load will be returned. Generally, the force returned will be a minor fraction of the original compressive load. Accordingly, energy resulting from heel strike is absorbed and dissipated and is not returned to the athlete""s body. The shock absorbers of the invention operate in a manner that is analogous to hydraulic dampening, as contrasted with springs that return applied compressive forces.
Desirably, the shock absorbers of the invention have plural stages, namely a relatively soft, easily compressible first stage, and at least one stiffer second stage to absorb higher compressive forces. More specifically, the first stage has a compression value that is predetermined to provide noticeable cushioning for low activity levels, such as walking. The second stage has a significantly higher compression value predetermined to absorb maximum loads from high activity levels, such as running or jumping. Additional or intermediate stages could be provided. There may be a multiplicity of incremental stages. As a result, the shock absorbers of the invention provide cushioned comfort at light activity levels, e.g., walking, while providing superior shock absorption at higher activity levels, e.g., running.
The stiffness of the shock absorbers may be adjusted as desired. Preferably the rear most shock absorbers are stiffer than the forward ones. In some applications, it may be desirable that the rear lateral shock absorber is stiffer than the medial shock to absorb anticipated heel strike on the lateral side. Further, the stiffness of the shock absorbers may be adjusted based on the anticipated compressive forces. By way of example, small shoe sizes should have relatively softer shock absorbers than large sizes. Midsoles for men""s footwear should have stiffer shocks than women""s. Midsoles designed for running applications should be softer than midsoles for basketball.
The midsole of the invention further comprises a semi-rigid, integral heel cup and shank member. The heel cup includes a semi-rigid upper plate. The member provides torsional rigidity and structural integrity to the midsole. A transverse opening or slot is provided between the heel cup and shank portions. The shank has a longitudinal, upwardly projecting arch that separates and bridges between the heel and forefoot portions of the sole. Thus, the shank supports the user""s arch and provides an improved appearance to the footwear.
The lower plate has a forward edge that projects through the transverse opening in the heel cup and shank member. The forward edge is attached to a top, inside surface of the shank. A forefoot midsole comprised of conventional ethylene-vinyl acetate (xe2x80x9cEVAxe2x80x9d) foam is provided. A rear portion of the EVA foam extends over the shank and fills and seals the opening in the upper member.
The lower plate is desirably composed of a material that is softer and more flexible than the upper plate. As a result, the top portions of the shock absorbers are fixed relative to one another, while the lower portion of the shock absorbers have some freedom of movement relative to one another. Thereby, the bottom surface of the footwear may flex to engage the contours of the ground while allowing the heel cup and foot of the wearer to remain relatively stable. The comparatively soft lower plate, anchored to the shank, allows the shock absorbers a smooth and full range of motion.
The shock absorbers are fasten to the upper and lower plates with adhesive and mechanical connection. The upper plate has a plurality of recesses, each recess for receiving and mounting a top surface of a respective one of the shock absorbers. A bottom edge of each shock absorber has a flange. The bottom plate has a plurality of brackets for receiving each shock flange to thereby mount the shock absorbers to the bottom plate.
A vertical heel stiffener is provided that is unitary with the lower plate and connected to the heel cup. The heel stiffener is disposed on the medial side of the heel, and as a result provides over pronation control.
The heel portion of the midsole of the invention is desirably open so as to expose the shock absorbers to view. The absence of sidewalls reduces weight. Further, exposure of the shock absorbers provides a visually enhanced look to the footwear. To further enhance the visual effect, the midsole of the invention may optionally comprise a light source attached to the heel midsole for illuminating the shock absorbers. The heel cup is preferably translucent and the light source comprises sheet material in the heel cup. Thereby the light source illuminates the heel cup and shock absorbers creating a desirable visual effect.
Accordingly, the objects of the invention have been achieved. Further features and advantages of the invention will become apparent from the detailed description of the preferred embodiment of the invention that follows.